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Hidden cosmic fuel tank found in infant galaxy cluster

Astronomers using the Atacama Large Millimeter/submillimeter Array (ALMA), along with complementary data from the Atacama Pathfinder Experiment (APEX), have discovered a surprisingly large reservoir of molecular gas in a protocluster of galaxies known as SPT2349-56. This protocluster, located approximately 12 billion light-years away, is a region of the early universe where a cluster of galaxies is just beginning to form.

Galaxy clusters are the largest structures in the universe, and understanding their formation is a major goal of scientists. Protoclusters like SPT2349-56 offer a unique window into this process, allowing astronomers to observe galaxies as they come together in a dense environment.

This new research, led by Dazhi Zhou of the University of British Columbia and in The Astrophysical Journal Letters, focuses on the molecular gas within SPT2349-56. Molecular gas, primarily hydrogen, is the raw material for star formation, which plays a critical role in galaxy evolution.

Key findings

• Missing gas: By comparing observations from ALMA's high-resolution configuration with lower-resolution data from ALMA's Atacama Compact Array (ACA) and APEX, the team found a significant amount of molecular gas that was "invisible" in the higher-resolution ALMA images. The ACA detected 75% more CO than the sum of individual sources detected in higher-resolution ALMA data.
• Extended reservoir: This missing gas isn't just a few faint, undetected galaxies. Instead, it appears to be a diffuse reservoir of gas spread throughout the protocluster.
• Fuel for starbursts: This hidden gas reservoir could be the key to understanding the intense star formation activity observed in SPT2349-56. The presence of so much extra gas extends the star formation fuel, meaning the overall depletion timescale will exceed 400 million years.
• Proto-intracluster medium: The team speculates that this extended gas might be the precursor to the hot, diffuse gas known as the intracluster medium (ICM) that fills mature galaxy clusters.

"This discovery highlights the power of ALMA, especially when used in multiple configurations," says Zhou. "The high-resolution observations allowed us to pinpoint individual galaxies, while the lower-resolution data revealed the bigger picture—the extended gas that connects these galaxies and fuels their star formation."

SPT2349-56 is an extreme system, producing stars ~10,000 times faster than our Milky Way but in a comparable size, and these observations have pushed scientists' understanding of galaxy formation and evolution. No simulation or galaxy formation model had previously predicted this overdensity of gas.

These findings also suggest that high-resolution ALMA observations, while excellent for studying individual galaxies, may miss a significant component of the gas in these early clusters. The missing gas may reside in the circum-galactic medium (CGM) or the pre-heated proto-intracluster medium (proto-ICM).

Future studies using ALMA's full capabilities, including its compact array configurations, will be crucial for fully characterizing this hidden gas reservoir and understanding its role in the formation of galaxy clusters.